It was back-to-school day for Marjory Stoneman Douglas High School in South Florida. Just six months ago, the school was the site of a mass shooting. A gunman killed 17 people and injured 17 others.

With the beginning of a new school year, there are some changes - new security measures and portable classrooms to replace the now-shutdown building where the shootings occurred. From Parkland, NPR's Greg Allen reports.

(SOUNDBITE OF WHISTLE BLOWING)

GREG ALLEN, BYLINE: Except for the reporters and TV cameras, it was like the first day of school anywhere - students happy to see old friends, bantering with crossing guards, getting off school buses. But junior Sara Cardona said coming just one day after the six-month anniversary of the shooting, it was tough.

SARA CARDONA: It's going to feel weird to not see some kids in our classes that we're used to seeing and stuff 'cause I feel like during summer, you don't really see anybody, so it's, like, OK. But I feel like a lot of people are hopeful and excited for the new year. And I feel like it's going to be OK.

ALLEN: A lot of work has been going on at the high school and throughout the district over the summer. The old freshman building where the shootings took place will eventually be torn down. A 12-foot fence surrounds it. The state has set aside $25 million for replacement. But for now, 34 portable classrooms will be used.

After greeting arriving students today, Broward Schools Superintendent Robert Runcie briefed reporters on the new security measures in place in Parkland and at schools throughout the district.

ROBERT RUNCIE: Armed security staff at each and every one of our schools. We continue to upgrade our surveillance camera systems at every school in Broward County. We are moving forward and accelerated our single-point-of-entry projects, which use a system of gates, fencing to funnel all visitors to a single point.

ALLEN: Instead of one school resource officer, there are now at least three armed guards on campus at Marjory Stoneman Douglas, plus 15 other security monitors. The district is also moving forward with a state-mandated guardian program that allows certain school employees to carry guns on campus.

Students and parents said they're satisfied with the new measures and believe their school's now one of the safest in the country. Andy Libert was there dropping off his son Ryan, a freshman.

ANDY LIBERT: They've stepped up security. And there's measures in place now that hopefully will stop it a little bit. But it could've been at a mall. It could've been anywhere. Bottom line is the kid fell through the cracks in the system, and he needed help.

ALLEN: The former student who police say confessed to the shooting is awaiting trial on 17 counts of murder. Prosecutors have indicated they'll seek the death penalty. Superintendent Runcie says the district is increasing the number of mental health counselors and behavior specialists and is upgrading its risk assessment teams.

RUNCIE: So when there's students who are identified who we know have challenges, we've got to make sure that we're doing the right kind of interventions, that the right kind of supports are there.

ALLEN: It's not just the schools that have changed since the shooting. Many of the students at Marjory Stoneman Douglas have now become activists, pushing for gun control and other measures they believe will make schools safer. Lauren Hogg spent her summer on the "March for Our Lives" bus tour through Florida and up to New York.

LAUREN HOGG: And now having to go back to school, it's like, I joke - like, this whole summer, I've been joking with my friends who are on the bus, like, there's three years until I can take a government class, but I'm lobbying in D.C.

ALLEN: Hogg says with all the security upgrades, she's not worried about safety at her school. But she says it doesn't begin to address the problem of gun violence.

LAUREN: What they did at my school is, in a way, putting a Band-Aid on a broken bone. Sure, it'll fix this, but how is it going to protect my friends who go to concerts, who go to clubs, who go to movie theaters, who go anywhere else and are in danger - even some of them just getting to school in inner cities?

ALLEN: Students had their school IDs and backpacks checked by security personnel when they arrived today. Plans to install metal detectors, though, were put on hold in part because of concerns about the logistics of having 3,000 students walk through them every day. Greg Allen, NPR News, Parkland, Fla. Transcript provided by NPR, Copyright NPR.